LLC converter is widely used in various applications due to its simple structure and zero voltage switching capability. To achieve high efficiency, LLC converter is usually designed to operate near the resonant frequency over the entire load conditions. However, a constant core loss of the main transformer due to the constant frequency operation has a bad effect on the efficiency in a light load condition where the core loss dominates the total loss. Therefore, it is essential to reduce transformer core loss in order to improve light load efficiency. Adapting asymmetric pulse-width modulation (APWM) in a half-bridge LLC resonant converter with voltage doubler rectifier for improving light load efficiency has been proposed. By adapting APWM, the core loss can be reduced dramatically in the light load condition, because both the switching frequency and the flux swing of the transformer are reduced. Furthermore, the proposed method does not affect the design of the converter. The experimental results with 400–250 V input and 100 V/2 A output prototype converter support the effectiveness of the proposed method.